The exhaust gases of a two-stroke engine of a motor-driven chain saw enter the catalyzer with an exhaust gas temperature of approximately 600.degree. C. A chemical conversion process takes place in the catalyzer where, for example, hydrocarbons are converted to carbon dioxide and water and this conversion process causes the temperature to increase in the catalyzer to approximately 1,000.degree. C. since the conversion process is exothermal. Energy-rich exhaust gases are available for the conversion process because of the scavenging losses which are typical for a two-stroke engine. During the conversion process, heat energy is released which corresponds approximately to the energy delivered by the engine.
Since the catalyzer is uncontrolled, a deficiency of air can not be excluded in component regions of the characteristic field of the engine so that a complete conversion of the noxious exhaust gas components of hydrocarbons and carbon monoxide can not be obtained in these regions. If these exhaust gases reach the ambient air containing oxygen through the exhaust outlet or through a gap at the partition interface of the muffler housing, the residual gas can ignite if the temperature downstream of the catalyzer is correspondingly high for the oxidation processes. This can be disadvantageous especially for handheld portable tools such as chain saws and especially when there is a switchover from high rotational speed and full-load operation to idle operation.